(1.) Field of the Invention
The present invention relates to a contact lens and more particularly to a novel contact lens of no water absorbability having excellent oxygen permeability.
(2.) Description of the Prior Art
Contact lenses are largely divided into contact lenses made of hard materials and contact lenses made of soft materials. As the contact lenses made of hard materials, those comprising a polymethyl methacrylate as a main component have conventionally been used widely, but have had a problem of low oxygen permeability and allowing no long continuous wear. Hence, in recent years, there were developed contact lenses with excellent oxygen permeability, such as those comprising a silicone monomer and those comprising a fluorine-containing monomer (e.g. trifluoroethyl methacrylate), as disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) Nos. 194014/1983 and 38418/1987. With these contact lenses, the disturbance of corneal metabolism is low and the long continuous wear is possible.
Meanwhile, as the contact lenses made of soft materials, those of low water absorbability comprising hydroxyethyl methacrylate as a main monomer component have conventionally been used widely, but similarly to the above hard contact lenses, have had low oxygen permeability and have allowed no long continuous wear. Hence, in recent years, there was developed a contact lens of high water absorbability comprising N-vinylpyrrolidone as a main monomer component. Being able to absorb a large amount of lacrima thereinto, the lens has improved oxygen permeability and allows long continuous wear.
As the contact lenses made of soft materials, there are also soft contact lenses of no water absorbability, besides the above-mentioned water-absorbable soft contact lenses. As an example, there can be mentioned a contact lens comprising a silicone rubber, as disclosed in, for example, Japanese Patent Publication No. 61122/1987. Having no water absorbability, this contact lens causes neither spreading of stains into the interior of contact lens nor propagation of bacteria, and accordingly requires no boiling sterilization by water. The lens is further characterized by good oxygen permeability.
As other example of the soft contact lenses of no water absorbability, there is a contact lens comprising a copolymer of an acrylic acid ester and a methacrylic acid ester, as disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 210318/1982. This lens causes no spreading of stains into the interior of lens because of no water absorbability similarly to the above case, but has inferior oxygen permeability and allows no long continuous wear. Therefore, in recent years, there was proposed a contact lens comprising a copolymer of (a) an acrylic acid ester or a methacrylic acid ester and (b) a fluoroalkyl acrylate or a fluoroalkyl methacrylate, as disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 229113/1987. By containing fluorine, this contact lens is designed to have improved oxygen permeability and enable higher oxygen supply to the cornea.
The above-mentioned hard contact lenses, although having significantly improved oxygen permeability, have drawbacks in that being a hard type, they give strong mechanical irritation to the cornea or the sclera when worn, give to the user an uncomfortable feeling of having a foreign object in the eye(s), and cause hyperemia and/or sore. When the extent of hyperemia and/or sore is high, it is impossible in some cases to wear these lenses, which is a big drawback of the hard contact lenses.
Meanwhile in the soft contact lenses, the mechanical irritation to the cornea or the sclera is small and accordingly good feeling is ordinarily obtained when wearing them, but they have the following problems. First, the problems of the water-absorbable soft contact lenses are described. The water-absorbable soft contact lenses take lacrima into the interior of lens; accordingly the lenses are easily stained by lacrima components; the stains easily spread into the interior of lens; and the lenses tend to become insanitary.
Further, the lenses require frequent boiling sterilization by water in order to prevent the propagation of bacteria in the interior of lens, which is a burden to lens users. Furthermore, since the staining of the water-absorbable contact lenses is higher as their water absorbability is higher, the cornea is placed in an insanitary condition over a long period of time and consequently may cause a corneal disoder, particularly when a soft contact lens of high water absorbability is continuously worn over a long period of time. This is a big problem of the water-absorbable soft contact lenses.
Next, the water-non-absorbable soft contact lenses have the following problems.
The contact lens comprising a silicone rubber, although having very superior oxygen permeability, adheres to the cornea, impairs the normal metabolism of the cornea, causes corneal disoder in some cases, thus is questionable as to the safety when worn. Therefore, it is hardly used at present.
The contact lens comprising a copolymer of an acrylic acid ester and a methacrylic acid ester has inferior oxygen permeability as mentioned above and accordingly allows no long continuous wear.
The contact lens comprising a copolymer comprising a fluoroalkyl (meth)acrylate as a monomer component contains said fluoroalkyl (meth)acrylate in order to obtain improved oxygen permeability. In this contact lens, however, the use of fluoroalkyl (meth)acrylate in too large an amount for higher oxygen permeability results in loss of lens flexibility, easy development of plastic deformation when the lens has been deformed, and poor shape recoverability.
The above lens further has water repellency at the surface, which causes a problem in wearing the lens. As a method for improving the water repellency, copolymerization with a hydrophilic monomer is affected generally. However, a copolymer of fluoroalkyl (meth)acrylate and a hydrophilic monomer has a general tendency of being cloudy, and even when being transparent, the copolymer gets cloudy in some cases when immersed in water. Thus, the copolymer has a problem in practical use.